This invention relates generally to two-way CATV communication systems and is specifically directed at improving subscriber upstream communication in a two-way CATV contention system particularly during periods of peak headend accessing.
In a two-way CATV system each subscriber is able to transmit signals back to the CATV headend. These subscriber-originated signals may include program purchasing requests, opinion poll responses, or CATV converter status information. The upstream signal distribution network is in the form of a "merging tree topology" in which the signals generated by many sources, or subscribers, converge and are transmitted on a single transmission line (the cable) back to the CATV headend. Collisions between upstream messages simultaneously transmitted by more than one subscriber result in the loss of the colliding messages and an unsuccessful communication attempt by the respective subscriber terminals.
In the two-way subscriber contention system referred to above various approaches have been taken for the purpose of increasing the rate of successful upstream message transmissions. One approach, generally termed "Aloha", involves the random transmission by those subscribers desiring shared media access. Simultaneous transmission of more than one subscriber results in the collision and mutilation of data packets and unsuccessful upstream communication. An unsuccessful transmission is typically followed by another random transmission by the unsuccessful subscriber who again awaits receipt of a response such as an acknowledgment from the headend indicating successful upstream message transmission. In order to further increase the rate of successful transmissions, the basic "Aloha" arrangement has been modified to form a so-called "slotted Aloha" arrangement wherein the headend transmits a synchronizing signal indicating the beginning of a frame which is divided into a sequence of time slots of mutually equal length. Each subscriber transmits a data packet to the headend within a given time slot which is randomly selected from the sequence of time slots. This approach is generally credited with reducing the number of data packet collisions to one half that encountered in the basic "Aloha" system. An example of the "slotted Aloha" approach can be found in U.S. Pat. No. 4,398,289 to Schoute.
Because of the unique environment of a two-way CATV communication system, systems utilizing either the "Aloha" or "slotted Aloha" approaches have suffered from performance limitations. For example, headend access in such systems is typically desired by most subscribers during relatively short peak periods in which a large number of subscribers transmit program requests shortly before the start of the desired program. At other times, relatively few upstream transmissions are made even in systems having large numbers of subscribers. Thus, refinements to the "Aloha" approaches have been made in order to accommodate this unique environment.
One approach to improving upstream data transmission in a two-way CATV contention system involves data packet collision detection. In this approach, each subscriber terminal is provided with a means for detecting data packet collisions in ascertaining upstream message traffic volume and for regulating subscriber terminal transmissions as a function of the detected message traffic volume. The Ethernet system is an example of this approach. Other systems make use of a so-called binary back-off algorithm for controlling the length of the window during which upstream data packet transmissions are made. For example, the length of the window for each subsequent upstream retransmission may be doubled until a predetermined window length is reached, at which time the subscriber terminal may either stop transmitting or begin the transmission cycle again starting with the shortest transmission window. This approach is graphically represented in FIG. 1 where transmission window size is shown in terms of the number of attempts or upstream data packet transmissions. This approach, which derives its name because the window size increases with the square of the number of attempts, is of limited efficiency in a two-way CATV contention system. For example, as the number of attempts approaches the predetermined cutoff point, each subscriber transmits within an increasing window even though the number of subscribers transmitting has decreased due to the successful transmissions of other subscribers who no longer contribute to the upstream data traffic load. Therefore, this approach does not address the unique environment of the typical two-way CATV subscriber contention system.
The present invention is therefore intended to overcome these and other limitations of the prior art by providing improved peak load headend access in a multi-subscriber CATV communication system wherein the subscriber upstream transmission rate is determined by the rate at which headend access is sought by the various subscribers.